Tornado infrasound: Observations and potential mechanisms

Tornado warnings have not improved over the past 20 years, which is especially true in hilly terrain where radar cannot see near the ground due to the curvature of the earth. While Tornado Alley is best known for tornadoes, most tornado related deaths occur in the southeastern US where hilly terrain is more prevalent. Tornadoes emit sounds at frequencies below what humans can hear (infrasound), and there is strong evidence that these sounds carry information about the forming of the tornado as well as its size. In addition, these very low frequencies can travel well beyond the line-of-sight. Currently, this information is not used to guide warnings because we do not understand what makes it. Our team has been working on identifying the fluid mechanism for the past few years and there are four commonly proposed mechanisms that are consistent with observations; radial oscillation, latent heat effect, pressure relaxation, and shear instability. In this presentation each mechanism will be discussed relative to available observations. In addition, an overview of current field and laboratory testing motivated by the success and limitations of each proposed mechanism will be discussed.